A fuel cell vehicle, such as a fuel cell electric vehicle, may use power generated by fuel cell stacks (FCS) to power an on-board electric motor and drive wheels of the vehicle. Fuel cells that make the FCS may be electrochemical devices in which oxygen and hydrogen react to generate electricity, water and heat. The electricity generated at the FCS then drives the electric motor of the fuel cell vehicle, while the water produced at the FCS serves to hydrate the membrane of the fuel cell. Any excess water flows out of the FCS. The heat generated at the FCS may be transferred out of the FCS and into the vehicle (e.g., the cabin space) via a cooling loop. However, at sub-zero or freezing ambient temperatures or during cold-start of the fuel cell vehicle, residual water remaining in the membrane from the previous operation of the FCS turns into ice, thereby blocking the pores in the cathode catalyst layer of the fuel cell. This ice accumulation in the membrane makes portions of the cathode catalyst layer inactive during cold-start of the fuel cell vehicle by blocking the flow of oxygen to the cathode catalyst. If more oxygen and hydrogen is pumped into the fuel cell stack to meet the vehicle operator torque demand and generate heat, more water is generated which may also freeze and render an even larger portion of the cathode catalyst layer inactive, thereby further stopping electrochemical reactions from occurring in the FCS. Further still, any heat that is generated in the FCS as a by-product of the electrochemical reaction is whisked away from the FCS through the cooling loop. The net effect is that more and more ice begins to form in the membrane of the FCS, eventually resulting in a total loss of power in the entire FCS, and the vehicle not starting up.
One example cold-start method for a fuel cell stack is shown by Yoshihito in JP 4830341. The approach utilizes valves to restrict the flow of coolants through the fuel cell stack during a cold-start. Specifically, by using very low coolant flow rates across the fuel cell stack during the cold-start, heat removal from the fuel cell stack is restricted. By also controlling a coolant pump operation, coolant supply to the FCS is reduced at low temperatures. The valves allow for other components in the cooling loop to maintain a higher coolant flow as may be advantageous to dissipate the power drawn from the fuel cell without overheating.
The inventors herein have recognized the above issues and interactions, as well as additional issues of flow restricting systems. As one example, the approach of Yoshihito creates highly non-uniform temperatures throughout the fuel cell stack and the connected coolant loop. Non-uniform temperatures across the fuel cell membrane may lead to uneven distribution of water and ice in the membrane. In particular, at regions where the fuel cell membrane is not sufficiently heated up, the water produced may quickly turn into ice, resulting in active area loss and may eventually lead to FCS shutdown. In addition, a low coolant flow in the FCS may cause overheating of some regions of the FCS. Furthermore, the solution requires valves, which add cost to the vehicle.
In view of these issues, the inventors have identified an approach to reduce ice build-up during start-up of a fuel cell stack under water freezing conditions. In one example, the issues described above may be addressed by a method for a vehicle comprising: during fuel cell stack start-up, limiting power drawn from a fuel cell stack based on a water solubility of the MEA of the fuel cell stack, and a temperature. Limiting the power drawn includes limiting a current density, and wherein the power limiting includes limiting electrical power drawn from the fuel cell stack based on a time to raise an inlet temperature of the fuel cell stack. In this way, the total amount of water (including water remaining and water generated) in the membrane can be controlled to be within an ice tolerance curve of the fuel cell stack.
In one example, the water content of the fuel cell membrane may be learned during a fuel cell stack shutdown and used to determine the water solubility in the MEA and further limit the power drawn from the fuel cell stack during a subsequent start-up when the ambient temperature is below a threshold (such as a threshold where the water may freeze). By knowing the amount of water that remains in the membrane, and limiting the amount of water that is generated as a by-product of the electrochemical reaction in the fuel cell stack (by limiting the power drawn from the fuel cell stack), the membrane may be operated within an ice tolerance curve. Limiting the power drawn from the FCS includes not drawing the full power but drawing only a fraction of the entire power that the FCS is capable of generating. By drawing limited power from the FCS, where the limit may be determined in real time during the start-up based on sensed temperature and water amount estimates, it may be possible to operate the FCS with reduced intermittent power loss due to ice formation in the membrane. Further, a coolant pump may be operated during the cold-start to return heated coolant at a higher flow rate thereby reintroducing heat back into the fuel cell stack more quickly and reducing the time period over which current is limited.
During freeze start-up of the vehicle, if full power is drawn from the FCS, more water is generated in the FCS. If sufficient heat is not returned quickly to the FCS, water may begin turning into ice and start blocking the pores. It may still be possible to draw power from the FCS, but the performance may begin to degrade and eventually the entire stack may freeze and no power may be generated leading to FCS shutdown. The inventors have recognized that by limiting the power during freeze startup, it may be possible to operate the FSU without any power loss and preempt FCS shutdown. The technical effect of limiting the power drawn from the fuel cell stack when ambient temperature is low is that the fuel cell stack produces less water, and therefore accumulates less ice. In one example, by concurrently directing heat back into the FCS by pumping coolant at a higher flow rate, heat may be quickly returned to the fuel cell stack and any ice in the membrane can be melted. As a result, active freezing of large areas of a fuel cell membrane during a cold-start is reduced. Overall, intermittent or total loss of power in a fuel cell stack during cold-start of a fuel cell vehicle may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.